Topological insulators (TI) form a new quantum phase of solid state matter distinct from the classic dichotomy of metals and insulators. Although the bulk is nominally insulating (forms a band gap), the two dimensional surface bands form a Dirac cone populated by massless fermions. These surface states are topologically protected against disorder scattering and are spin-polarized. Hence TIs are expected to produce new functionalities for a wide range of applications such as low power electronics (owing to dissipationless transport), spintronics (utilizing spin-polarized current), and quantum information technology.
One of the unique and most striking properties is that of spin-momentum locking—the spin of the surface state is locked at right angle to the carrier momentum. This characteristic has been probed by photoemission measurements on cleaved samples in ultra high vacuum.
In principal, inducing a net carrier momentum (by applying a bias current, for example) spontaneously generates a net spin polarization.
However, attempts at accessing the spin-momentum locking of the surface states by electrical means, i.e., generating such spin polarization and directly sampling the current-induced spin polarization, have not been successful, and these effects have not been demonstrated outside of our work reported here.
This in large part is due to the fact that the bulk is typically heavily doped, and therefore provides a parallel conduction path, which short circuits transport in the surface states and overwhelms any signal originating from the surface states.
Here, by utilizing a ferromagnet/tunnel barrier contact that is intrinsically sensitive to surface/interface spins, demonstrated for the first time is that one can directly generate a net spin accumulation with a simple bias current, and directly detect the current-induced spin polarization of the surface spin due to spin-momentum locking, where its projection onto the detector contact magnetization manifest as a voltage.